An SSL/TLS certificate does two things: data between the browser and the site is encrypted (passwords, forms), and the browser verifies it’s really talking to the right domain. Without one, browsers mark a site as “not secure”.
Certificates expire, so renewal should be automated. Let's Encrypt issues free certificates that renew automatically — often handled by the reverse proxy.
Example
A certificate expired overnight and the site showed “not secure” — after enabling automatic renewal it doesn’t happen again.